The present invention relates to a generator that produces both heat and electricity from solar energy, and in particular to a solar energy co-generator that utilizes a transfer fluid to capture heat energy and increase the efficiency of photovoltaic generation.
Solar energy is a renewable energy source with many well-known advantages over traditional fossil and nuclear energy resources. Solar energy has remained a largely untapped field, however, because the efficiency of existing solar power collection systems is generally too low to justify the required capital investment for these systems. While most of the electromagnetic radiation received from the sun falls in the visible spectrum (a wavelength range of approximately 430 to 690 nm), the sun's radiation also includes other components. In particular, solar energy incident upon the earth includes infrared radiation, a band that covers a wavelength range of approximately 1 μm to 1 mm. Typical solar cells, that is, devices designed to convert electromagnetic radiation from the sun into electricity, are only able to utilize energy within a relatively narrow wavelength band in the visible spectrum. Infrared radiation is not only unusable by such solar cells, but the heat caused by incident infrared radiation actually lowers the efficiency of typical semiconductor-based solar cells. Existing thermophotovoltaic cells, which are designed to capture infrared radiation and convert it to electricity, have very low efficiencies and are thus limited to a small number of specialized applications, such as recapturing infrared radiation emitted by heated surfaces. In addition, existing solar cells generally convert longer wavelength visible light into electricity more efficiently than they do shorter wavelength radiation. As a result of these limitations, much of the electromagnetic energy incident on a typical solar cell cannot be converted into electricity. It would be desirable therefore to develop a method of efficiently converting a broader band of the sun's radiation to usable energy, thereby increasing the efficiency of a solar energy collector and making such systems practical solutions for a greater number of energy applications.
Prior art systems have been developed to capture heat energy (that is, energy resulting from incident infrared radiation) from the sun. For example, U.S. Pat. No. 6,336,542 to Tirey, Jr. discloses a water heating system utilizing a reflective parabolic dish as a solar concentrator. A fluid line extends through a support pole and is coiled at the focal point of the dish. Solar energy, which is concentrated at the fluid line coil by the reflective dish, heats water that is pumped through the system. Another example, disclosed in U.S. Pat. No. 5,685,151 to Ross, is a system that utilizes a solar concentrator to capture solar energy and focus it onto a cavity-type solar boiler. The interior walls of the boiler are formed by pipes that carry liquid sodium, which acts as a heat transfer medium. The heat causes the sodium to boil, and the sodium vapor is pumped to a heat exchange system in communication with a molten sodium chloride tank. The sodium returns to a liquid state as a result of the heat exchange, and is pumped in a circuit back to the boiler. The heat now stored in the sodium chloride tank is then transferred again by steam pipes as needed and used to power electrical generators. Similarly, U.S. Pat. No. 4,586,334 to Nilsson, Sr. et al. discloses a system for collecting solar power with a concentrator and storing it in a phase change medium, whereby energy may be withdrawn on demand using a variable heat exchange system. U.S. Pat. No. 5,228,293 to Vitale discloses a system that captures solar energy in a cavity receiver and transfers it through a thermal transport fluid to an engine that may produce both electrical power and hot water. The system also includes an auxiliary fossil or biomass heater that may supplement or completely replace the cavity receiver.
There have also been attempts to develop solar energy co-generators, that is, solar energy collection systems that capture energy from both the visible and infrared spectrums, utilizing a combination of solar cells and heat transfer fluids, to produce both electricity and heat energy. Such systems should in theory operate with greater efficiency than systems designed to produce heat or electricity alone since a wider portion of the sun's spectral band is utilized. Such systems would have important practical applications, such as using electricity to power electrical appliances, and heat for home heating or hot water production. An example of an attempt to design such a system is disclosed in U.S. Pat. No. 6,080,927 to Johnson. The Johnson system includes a solar concentrator that focuses sunlight onto the solar cells. A heat transfer system is placed in contact with the solar cells in order to carry away heat resulting from the infrared radiation incident upon the solar cells. The heat transfer system is simply water or the like pumped through pipes in contact with the solar cells, with a heat exchanger that transfers heat from the heat transfer system to some useful work, such as heating a swimming pool. By carrying heat away from the solar cells, this system allows the solar cells to operate more efficiently. A further example is provided by U.S. Pat. No. 4,191,164 to Kelly, which teaches a system with a solar concentrator that provides solar energy to pre-heat a flash boiler pipe for steam production. The system also includes an array of solar cells mounted at the cold water entrance end of the pipe. The cold water serves as a thermal transfer fluid that dissipates heat from the solar cells as in Johnson, while the infrared radiation is used for pre-heating the boiler pipe water.
Prior art co-generation systems such as Johnson and Kelly suffer from some significant limitations and disadvantages. One critical problem with these systems is that while they may include a heat transfer medium to draw heat away from the solar cells, they do not prevent the heating of the solar cells by infrared radiation. Since these systems typically include a solar concentrator, the heat generated at the solar cells may be quite intense. Although the use of a heat transfer system in contact with the cells does provide some cooling, the efficiency of the solar cells is still reduced simply by the fact that the infrared radiation reaches the cells at all. A system whereby a significant part of the sun's infrared radiation never reaches the cells, but is instead directly converted to heat energy before it reaches the cell surface, would be inherently more efficient.
One system that attempts to address this problem is taught by U.S. Pat. No. 6,407,328 to Kleinwächter. Kleinwächter teaches that a liquid medium that permits passage of radiation energy useful for photovoltaic effects, but absorbs the remaining radiation energy, would be desirable. Kleinwächter attempts to achieve this by the use of water in combination with a fluoropolymer film. Kleinwächter teaches that water absorbs only the longer-wave radiation and allows passage of wavelengths that may be useful for photoelectric production, while the fluoropolymer film blocks only a portion of the short-wave range. As a result, most radiation in the band that is useful to solar cell electricity production passes through while the longer-wave radiation heats the water passing over the solar cells. Kleinwächter further teaches a heat exchange system for cooling of the water.
A significant limitation of the system taught by Kleinwächter is that it provides no means by which to disperse radiation uniformly across the solar cell. Solar cells reach their maximum efficiency only when radiation is evenly distributed across their functioning surface. Providing a uniform distribution of light across a solar cell is particularly difficult where a solar concentrator is employed as part of the collection system. One method that has been utilized to alleviate this problem is to employ a luminescent material between the solar cell surface and the sun. Luminescent materials have the property that they may absorb light falling upon them from one direction and reemit light that is isotropic. As a result, a luminescent material may be constructed that will “smooth” the intensity distribution of incident sunlight across a surface, such as a solar cell. An example of a system that attempts to utilize this principle is taught in U.S. Pat. No. 4,175,980 to Davis et al. Davis et al. teaches that luminescent materials may be suspended in a sheet of glass or plastic positioned above a solar cell in order to smoothly distribute light across the solar cell.
The use of a luminescent material as described above may also increase the efficiency of a solar cell by producing light of a frequency that is well matched to the performance characteristics of the solar cell. Roughly ten percent of the sun's electromagnetic energy falls in a wavelength range of about 305 to 450 nm (that is, from near ultraviolet to blue). Typical semiconductor solar cells, however, operate more efficiently at somewhat longer wavelengths, toward the red end of the visible spectrum. In addition to their isotropic scattering property, certain luminescent materials also act to absorb visible light at the higher frequencies and emit light at the lower, more desirable frequencies. Thus the deployment of a luminescent material between a solar cell and the sun (or a concentrator) may serve to increase efficiency not only through more efficient distribution of incident light upon the solar cell but also through frequency conversion. U.S. Pat. No. 4,135,537 to Blieden et al. teaches a light collector that attempts to take advantage of this idea. Blieden et al. teaches that a luminescing fluid is deployed over the solar cell surface. This luminescing fluid serves to isotropically distribute light across the solar cell surface and also acts as a heat dissipation medium. Blieden et al. also teaches, however, that the luminescing agent may be chosen for its ability to emit light in an energy level range that suits the conversion characteristics of the particular solar cell being used.
The device taught by Blieden et al. also has significant limitations. Importantly, this device depends entirely upon luminescent materials to isotropically distribute light. As Blieden et al. discloses, this is not truly isotropic scattering, but is rather isotropic re-emission. In other words, the fluid taught by Blieden et al. does not simply scatter incident light in all directions, but instead absorbs the incident light (by means of luminescent materials in the fluid) and re-emits longer wavelength light isotropically. Although Blieden et al. claims that the invention is directed to the use of any luminescent material, in fact it teaches the use of only one class of such materials, namely, organic dyes. Organic dyes degrade with use, and this degradation is hastened by the absorption and re-emission as taught by Blieden et al. Furthermore, this degradation would be even further hastened by a higher concentration of incident light reaching the luminescent material, such as would be the case if one attempted to combine the Blieden et al. device with a solar concentrator. The constant replacement of transfer fluid material would render the device of Blieden et al. impractical in many-real-world applications, particularly with respect to applications that make use of a solar concentrator.
Another limitation on the use of organic dyes in such applications is self-absorption. These dyes not only absorb light in the shorter wavelength bands but in the longer wavelength bands as well. Thus the dye material could, for example, absorb incident light, re-emit light at a longer wavelength, and then re-absorb the emitted light. This re-absorption lowers the efficiency of the system and would hasten the degradation of the organic dyes.
Finally, the prior art includes the use of anti-reflective coatings to prevent the loss of efficiency due to reflective losses at the point of light entry into a solar cell. Materials such as zinc sulfide, an inorganic phosphor, have been used for this purpose, due to this material's high index of refraction. In a co-generator where a fluid is disposed above a solar cell, a protective coating must be provided between the solar cell and fluid in order to protect the solar cell from damage. This coating material is typically glass. The anti-reflective coating on the solar cell is typically matched to the refractive index of the protective coating in order to minimize losses. One important limitation of such prior art anti-reflective coatings, however, is that they provide no means by which to modify the wavelength of light passing into the solar cell in order to increase the solar cell's efficiency.
It would therefore be desirable to develop a solar co-generator with a long-life transfer fluid that blocks longer-wave radiation from reaching the solar cells and also converts that radiation to heat energy, while simultaneously providing isotropic scattering (rather than simply isotropic re-emission) and selected wavelength modulation with respect to the light reaching the solar cell. Further, it would be desirable to employ with such a co-generator an anti-reflective coating on the solar cells that serves to further provide wavelength modulation to increase the efficiency of the solar cells. Finally, it would be desirable to create such a solar co-generator that could be used with a light concentrator without significantly reducing the life of the transfer fluid used in the system.